1. Field of the Invention.
The present invention relates to a printhead carrier in an ink jet printer, and, more particularly, to a linear motion transmission device for a printhead carrier in an inkjet printer.
2. Description of the Related Art.
With inkjet printer products, a printing mechanism containing one or more printheads 10 (FIG. 1) must be traversed across the printing surface 12. The printing mechanism is supported and slides along two smooth, round support shafts 14, only one of which is shown. The force F needed to propel the printing mechanism is applied with a toothed belt 16 attached to the printing mechanism. Toothed belt 16 is then wrapped on a pulley 18 connected to a motor that applies torque. Disadvantages of this printing mechanism system include sensitivity to the belt attachment point, uneven drive force from the motor and belt system, and high friction printing mechanism supports.
Due to the small length of an inkjet printhead, the printhead 10 must be moved in some fashion over the entire width of a piece of media 12. This is usually accomplished by traversing printhead 10 across the width of the media 12, moving the media 12 lengthwise and repeating the process until the entire piece of media 12 has been covered by printhead 10. Other methods exist, but the same principle applies.
In order for the printhead 10 or, in a color printer, group of printheads 10 to traverse, a force F must be applied to printheads 10. These printheads 10 may be contained in a carrier 20. In most inkjet printers today, belt 16 is attached to carrier 20 and applies a force F causing it to traverse. Other techniques include the use of a leadscrew, toothed rack and pinion, or linear stepper motor.
The attachment point of belt 16 should be located at the center of gravity of the carrier mechanism to prevent any undesired rotational moment forces in carrier 20. Moment forces in carrier 20 will cause the carrier mechanism to rotate, changing the relationship between the printheads 10 and media 12. This change will cause print quality defects and increase friction in the carrier supports 14 to counteract the moment forces.
Printheads 10 are directly attached to an ink tank on carrier 20. As printing operations are conducted, the amount of ink present on carrier 20 is reduced. The change of ink mass causes the center of gravity to change. Since the center of gravity changes and the belt attach remains fixed, a moment force will begin to appear as the volume of ink in the tank is reduced during printing.
One known method to counteract this moment force is to create tighter supports 14 for carrier 20. Constraining carrier supports 14 could protect carrier 20 from undesired moment forces by only allowing motion along the axis of carrier supports 14. Unfortunately, tighter supports 14 result in a higher level of friction. More force will be required to move carrier 20, and accurate motion will not be achieved due to the increased static and kinetic coefficients of friction.
To apply accurate force quickly to obtain fast carrier accelerations, a toothed belt 16 is typically used. The tooth belt 16 introduces error into the system due to the belt teeth engaging and disengaging on drive pulleys 18. This added cyclical error often results in print quality defects.
To improve on the stated limitations and problems with the belt drive system, a leadscrew assembly could be used to provide a traverse force on the carrier. The leadscrew assembly consists of a machined screw and a nut that encompasses a portion of the screw. As the screw rotates, the rotationally constrained nut moves along the screw. As the screw is rotated, the nut slides along the screw threads, but suffers from drag due to friction. Accuracy is limited to the screw profile. Another problem is that accurate leadscrews are typically cost prohibitive.
What is needed in the art is an inexpensive and accurate linear motion transmission device for a printhead carrier of an ink jet printer.
The present invention provides a continuous linear motion transmission device that can smoothly, accurately, and inexpensively traverse a printhead carrier using roller bearings that engage and are angled relative to the carrier support shaft.
The invention comprises, in one form thereof, a printhead carrier assembly in an ink jet printer. The assembly includes a carrier moving along a linear path. At least one rotatable bearing is attached to the carrier. The at least one rotatable bearing has at least one axis of rotation. A rotatable shaft has a surface in contact with the at least one bearing such that the shaft is nonparallel to the at least one axis of rotation of the at least one rotatable bearing. The shaft is substantially parallel to the linear path of the carrier. Rotation of the shaft causes the at least one rotatable bearing to roll along a helical path on the surface of the shaft to thereby carry the carrier along the linear path.
A continuous linear motion transmission device (CLMTD) is applied to transport the printing mechanism across the printing surface. This device provides smooth and even actuation force to the printing mechanism and acts as a pure rolling support for the printing mechanism. The CLMTD attaches to one of the smooth support shafts and converts rotary motion of the smooth support shaft to linear motion of the printing mechanism.
An advantage of the present invention is that it provides very smooth and accurate linear motion due to the rolling support, low friction, and the ability to limit the force applied to the printing mechanism.